1. Field of the Invention
The present invention relates to an image processing apparatus and method, and to an image forming apparatus, and more particularly, to digital half-toning technology suitable for an image forming apparatus, such as an inkjet recording apparatus, which represents tonal gradations by the arrangement of dots.
2. Description of the Related Art
An inkjet recording apparatus forms a desired image on a print medium by ejecting ink droplets from nozzles while moving a recording head in which a plurality of ink ejection ports (nozzles) are arranged, and a print medium, relatively with respect to each other. In order to achieve a high-definition image output, the recording head used is a head with a two-dimensional nozzle arrangement in which the recording resolution is raised by means of a nozzle layout in which a plurality of nozzles are arranged two-dimensionally (the head may be also called a “two-dimensional head”).
Furthermore, in order to achieve good printing productivity, a composition for a single-pass image formation method is known, in which an image of a prescribed resolution is formed on a print medium by using a long line head (called a page-wide head or a full line-type head) having a nozzle arrangement which covers the whole range of the image formation region in the width direction of the print medium (hereinafter called the “x direction”) which is perpendicular to the conveyance direction of the medium (hereinafter called the “y direction”), and by adopting a composition in which this long head is arranged substantially perpendicularly with respect to the conveyance direction (y direction) of the print medium and the print medium is moved relatively just once with respect to the head.
In an inkjet recording apparatus based on a single-pass image formation method using a two-dimensional head of this kind, when high-speed printing is carried out, the landing time difference between the adjacent dots is short, and therefore the deposited droplets which have not been absorbed into the print medium combine together on the print medium, causing the granularity of the image to become worse. This issue of granularity is particularly marked in shadow areas.
Furthermore, normally, in inkjet printing, image formation is carried out by a multiple scanning method (multi-pass printing method) which completes an image of a prescribed resolution by means of a plurality of image formation scanning actions performed with respect to the image formation region on the print medium.
Forming the head to a long dimension is an effective means for achieving high productivity, and also it is desired to reduce the number of heads to a minimum from a cost viewpoint. For example, it is necessary to avoid compositions which are provided with a plurality of nozzles capable of recording dots onto the same pixel position on the print medium (nozzle redundancy), and to reduce the extent of overlapping of the heads. However, reducing redundant nozzles and the extent of head multiplexing leads to possibilities that the ejection characteristics of the individual nozzles are liable to be reflected in the image formation results, and band-shaped non-uniformities arise due to fluctuation between the nozzles.
In other words, in order to achieve high productivity at low cost, it is important to achieve both good granularity and reduced band-shaped non-uniformities, from the viewpoint of image quality. In general, it is beneficial to raise the dot size so as to cover up the gaps between dots, in order to improve banding. However, there is a possibility that increasing the dot size leads to worse granularity, and the problem described above (achieving both good granularity and reduced banding) cannot be resolved. Moreover, increasing the dot size involves a problem of increased ink volume. Increasing the ink volume means a requirement for increasing the drying properties, especially in the case of high-speed printing, which leads to higher costs in terms of increased capabilities of the drying apparatus, and the like.
In Japanese patent application publication No. 2006-82496, in order to resolve the problem of banding, technology is disclosed which suppresses band-shaped non-uniformities by modifying the image formation pattern, rather than controlling the dot size. In other words, a half-toning technique which changes the ratio of a band-shaped pattern and a distributed pattern is proposed.
As described in Japanese patent application publication No. 2006-82496, when a band-shaped pattern is used, the dots are liable to join together on the print medium, and therefore improvements can be definitely made in terms of banding. However, if patterns of two types, namely, a band-shaped pattern and a distributed pattern are simply combined, then there is a possibility that granularity becomes worse.
Furthermore, Japanese patent application publication No. 2006-82496 does not take account of the worsening of granularity as a result of combination of adjacently deposited droplets which can occur in the case of high-speed printing.